1. Field of the Invention
This invention relates to a turbocharger for an internal combustion engine. More particularly, this invention relates to a turbocharger including a symmetric twin-volute turbine housing having a nozzle ring with fixed vanes.
2. Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's power without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of power as larger, normally aspirated engines. Using a smaller engine in a vehicle decreases the mass of the vehicle, increasing performance and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which reduces emissions.
Generally, turbochargers use exhaust gas from an exhaust manifold to drive a turbine wheel, which is housed within a turbine housing. The turbine wheel and turbine housing define a turbine or turbine stage of the turbocharger. The turbine wheel is secured to one end of a shaft and a compressor impeller is secured to another end of the shaft such that rotation of the turbine wheel causes rotation of the compressor impeller. The compressor impeller is housed within a compressor housing. The compressor impeller and compressor housing define a compressor or compressor stage of the turbocharger. A bearing housing couples the turbine housing and the compressor housing together. The shaft is rotatably supported in the bearing housing. As the compressor impeller rotates, it draws in ambient air and compresses it before it enters into the engine's cylinders via an intake manifold. This results in a greater mass of air entering the cylinders on each intake stroke. Once the exhaust gas has passed through the turbine wheel, the spent exhaust gas exits the turbine housing and is usually sent to after-treatment devices such as catalytic converters, particulate traps, and Nitrogen Oxide (NOx) traps before exiting to atmosphere.
The turbine converts the exhaust gas into mechanical energy to drive the compressor. The exhaust gas enters the turbine housing at an inlet, flows through a scroll or volute, and is directed into the turbine wheel located in the center of the turbine housing. After the turbine wheel, the exhaust gas exits through an outlet or exducer. The exhaust gas, which is restricted by the turbine's flow cross-sectional area, results in a pressure and temperature drop between the inlet and outlet. This pressure drop is converted by the turbine into kinetic energy to drive the turbine wheel. Energy transfer from kinetic energy into shaft power takes place at the turbine wheel, which is designed so that nearly all the kinetic energy is converted by the time the exhaust gas reaches the turbine outlet.
In order to optimize the flow of exhaust gas to the turbine wheel, it is well known to include a nozzle ring which includes a series of curved vanes on a flange which form nozzle passages leading from the volute to the turbine wheel. The nozzle ring is sandwiched between the bearing housing and the turbine housing and the vanes direct the exhaust gas against the turbine wheel at an optimum angle.
Exhaust gas recirculation (EGR) is widely recognized as a significant method for reducing the production of NOx during the combustion process. The recirculated exhaust gas partially quenches the combustion process and lowers the peak temperature produced during combustion. Since NOx formation is related to peak temperature, recirculation of exhaust gas reduces the amount of NOx formed. In order to recirculate exhaust gas into the intake manifold, the exhaust gas must be at a pressure that is greater than the pressure of the intake air. However, if the pressure of the exhaust gas is excessive, the exhaust gas creates backpressure on the engine that is detrimental to overall fuel efficiency and performance.
One approach for ensuring sufficient exhaust gas pressure to promote EGR, while preventing excessive backpressure on the engine, is to use an asymmetric twin-volute turbine housing which incorporates two volutes of different sizes for separate exhaust gas routing of different cylinder groupings. A smaller volute coupled to a first cylinder grouping achieves EGR through higher exhaust gas backpressure built-up in front of the turbine. A larger volute coupled to a second cylinder grouping provides a high turbine output using exhaust gas energy for optimum efficiency without being affected by the EGR. This combination provides optimum engine response and helps the engine to comply with global emissions standards while achieving better fuel economy and improved performance.
It is understood, however, that multiple designs of the asymmetric twin-volute turbine housing are necessary to meet the desired EGR and turbine performance parameters depending on the particular application.
It is desirable, therefore, to provide a symmetric twin-volute turbine housing which can be used with multiple nozzle rings to effectively create an asymmetric twin-volute turbine housing with the desired EGR and turbine performance parameters.